Not all altimiters shunt when they are off. In the past there have
been problems with ematches blowing when people reversed polarity
going into their electronics, power oscillation caused by twisating
wires together instead of using a switch & bumping the rocket among
others. Shunting is not a bad idea although I think most people don't
I don't think I've ever heard of an ematch being set off by RF but, I
have heard people mention the possibility many times.
Also, it sucks if you forget to remove the shunt before flight.
Please think very hard before you use a key switch. They require keys
to operate, right? What happens if the rocket lands far enough away (or
is lost) and someone else finds it before you get to it with the key?
What happens if there is a live charge left in the rocket?
Use something that is easily operated with common tools (a screwdriver,
for example). Label it, so that e.g., if your rocket is hanging from a
powerline, the lineman can disarm it before fooling with it.
Also, make sure the switch you're using is both rated for the current
it will be drawing, and mechanically able to withstand the vibration it
will encounter. A surplus mil std microswitch, securely mounted, and
operated by a bolt inserted into the airframe, can work really well.
Standard practice is to switch the positive wire, isn't it?
Read the manual that comes with whatever electronic device you buy.
You might also read the manuals of the devices you are interested in
before you buy one so you have an idea of how the various devices
Ask people at the launch. Most people are happy to answer questions
and give advice.
Wasn't something changed here this spring for ALL electronic deployment
flights in the HPR safety code revision as a result of the Safety Committee.
NSL Muncie, a BP charge went off in someones prep area.
What increased the danger of an ematch is not its connection to an
altimeter, but it's proximity to a BP charge. Once in a container with
several grams of BP, you have to treat them VERY differently.
RF in and of it's self can not set off an ematch... What can possibly
happen: Extreme RF can cause transitory voltage to develop in some
electronic circuits and the transitory voltage can then cause triggering
of a firing circuit that would then in turn, fire the ematch. Although
that is in theory, I have never known a case of this happening, with any
of the devices we use. On the other hand, there is several altimeters
that have or, allegedly have, a history of firing because of "switch
bounce". Because of that problem I don't skimp on switches. I use either
a parallel wired DPDT slide switch or a screw type arming switch to
power up or down, all altimeters, Missleworks, Transolve, PerfectFlite,
and yes even Adept. I sometimes use shunts with bigger charges because
it makes me feel better, no other logical reason. I also cut the shunts
before powering-up the altimeter unit as I don't like the idea of the
unit circuit first reading 0 resistance, (dead short), and then +
resistance from the ematch if I cut the shunt after power-up. Once
again, no real reason, other than this "does seem logical".(:-)
A couple years ago I was prepping for a flight. I had recently added a
switch inside my ebay to turn on or off the Perfectflight MAWD altimeter. I
was testing the altimeter power switch. The events went like so...
I turned the switch on. (Insert screw). The Altimeter started beeping away
indicating all good and there was continuity to the ematches. Then I turned
the altimeter back off. (Remove screw). The altimeter didn't turn off. I
used the wrong screw and the switch got mashed and therefore was jammed in
the on position.
Now I have a rocket that has charges armed and ready to go off. Well I have
a couple other friends and flyers next to me as I'm doing this and after
trying to bump the switch back to the off position I finally decide the only
way I can get the power off now is to disassemble the rockets electronics
bay and remove the power source or power lead from the battery. As I go to
pull apart the rocket I unknowingly put my hand over the altimeter vent.
I must say I and my friends are all really glad I do a lot of ground testing
and fine tune my ejection charges rather than using the blow it out or blow
it up idea because as I closed my hand over the port, POP. The drogue charge
goes off. The bottom of the 12 pound rocket goes slamming into the ground at
my feet. Now I'm a little dazed as to why that just happened but I figured
it out as I was setting the rest of the rocket down and I take my hand off
the vent hole. POP! the main charge goes off this time. The nose cone
narrowly misses my friend!
I learned several things that day.
#1 Screw switches should be removed to arm not installed to arm.
#2 All ematches should be shorted until the rocket is on the pad and ready
for the igniter.
#3 Never cover a barometric vent hole
#4 Keep the pointy end up
I changed my set-up after that incident. Now I have a new set up using
switches that will short up to 4 ematches individually. As long as the
screws are through the airframe holding down the switch the altimeter will
not get continuity and the ematches are shorted so they are safe from static
and RF energy. I also use a separate connection or switch to arm the
Humm,, thinking on this one.
Shunting could damage the FET on an altimeter if not done right. then the
altimeter will act like it works, but never fire the charge, resulting in a
Almost all incidents I have seen of many charges going off wrong, shunts
would not have helped.
They either would fire the charge when the shunt was pulled, or fried the
altimeters FET and resulted in a ballistic recovery instead.
now we need to determine what's worse, a charge going off or a ballistic
Why were you arming the charges before the rocket was on the pad ?
If you wanted to check ebay setup with the ematches, do that before you you
ever add BP.
Once you ad BP, don't turn on or arm anything until you get the rocket
mounted on the pad.
I'm not sure that was the right lesson. The screw switches are a great
idea, but they have to be engineered properly, just like everything
If your rocket floats away, or lands on a powerline, how is someone
going to disarm it? Even if you label the holes "Insert 4-40 x 2" screw
here" who is going to have one of those?
Better to figure out a way to keep the switch from jamming, or only
carry one size of screw, or color-code the screws, than to have a
rocket that needs to be disarmed and have someone not know how to do
No I have not checked out Marks Modern High Power Rocketry 2. I did not
know about it. But thanks to you I can now. I hope you have a good launch
this am. I won't be there. I am a church goer so thats where I'll be
sitting. Have fun though. See you at the next saturday launch.
THe FET will not be damaged by shunting under NORMAL circumstances.
IF the outputs of the FET are shunted AND it attempts to fire the
charge they can be but they are not certain to be. So the potential
FET damage may possibly occur under conditions where you are trying to
prevent charges froom going off / trying to insure safety. Seems like
worthwhile to me.
Sorry, but "never" seems to be a strong term.
At the time I was looking into my L3, it was a requirement. And to me, one
that was foolish. I did my l3 with TRA. This was back when Lubliner (the
head of the L3CC) was still trying to get his own L3 cert.
I was there and know the history.
The discussion was about RF causing an ematch to "go off unexpectedly".
Yes, I've seen ematches go off unexpectedly, but ALL were caused by flier
errors. Your LDRS example just proves the case. A shunt or open would not
have changed what happened at LDRS, as the rocket was being armed at the
There was a bozo in my club that wanted to check out his electronics, and he
took his rocket apart to ask questions and then put it back together.
"Proof", or more correctly "Boom", as he didn't use a power switch, and the
change in pressure triggered the ejection charge, withing a few feet of
I was asking about RF and ematches.. Without a doubt, as that's what
"shunting" was to prevent.
There's nothing that will prevent a flier from screwing up his/her wiring
and having an ematch trigger at the wrong time, including shunts/opens.